Embedded Systems An Overview. This Week in Dig 2 Embedded systems overview What are they? Design challenge – optimizing design metrics What is.

Embedded SystemsAn Overview

This Week in Dig 2

Embedded systems overview What are they?

Design challenge – optimizing design metrics What is it that we need to be concerned with designing embedded systems?

Technologies Processor technologies IC technologies Design technologies

Embedded systems overview

What is an embedded system?

? Most of us think of “desktop” computers

PC’s Laptops Mainframes Servers

But there’s another type of computing system Far more common...

Embedded systems overview

Embedded computing systems Computing systems embedded within electronic

devices Hard to define. Nearly any computing system other

than a desktop computer Billions of units produced yearly, versus only (!)

millions of desktop units Perhaps 50 per household and per automobile

What are some of the examples of embedded systems?

Computers are in here...

and here...

and even here...

Lots more of these, though they cost a

lot less each.

A “short list” of embedded systems

And the list goes on and on

Anti-lock brakesAuto-focus camerasAutomatic teller machinesAutomatic toll systemsAutomatic transmissionAvionic systemsBattery chargersCamcordersCell phonesCell-phone base stationsCordless phonesCruise controlCurbside check-in systemsDigital camerasDisk drivesElectronic card readersElectronic instrumentsElectronic toys/gamesFactory controlFax machinesFingerprint identifiersHome security systemsLife-support systemsMedical testing systems

ModemsMPEG decodersNetwork cardsNetwork switches/routersOn-board navigationPagersPhotocopiersPoint-of-sale systemsPortable video gamesPrintersSatellite phonesScannersSmart ovens/dishwashersSpeech recognizersStereo systemsTeleconferencing systemsTelevisionsTemperature controllersTheft tracking systemsTV set-top boxesVCR’s, DVD playersVideo game consolesVideo phonesWashers and dryers

Some common characteristics of embedded systems

Single-functioned Executes a single program, repeatedly

Tightly-constrained on design metrics Low cost, low power, small, fast, etc.

Reactive and real-time Continually reacts to changes in the system’s environment Must compute certain results in real-time without delay

An embedded system example a digital camera

Microcontroller

CCD preprocessor Pixel coprocessorA2D

D2A

JPEG codec

DMA controller

Memory controller ISA bus interface UART LCD ctrl

Display ctrl

Multiplier/Accum

Digital camera chip

lens

CCD

• Single-functioned -- always a digital camera

• Reactive and real-time -- only to a small extent.

• Tightly-constrained -- Low cost, low power, small, fast

capture, compress, store, decompress, display

Design challenge – optimizing design metrics

Obvious design goal: Construct an implementation with desired functionality

Key design challenge: Simultaneously optimize numerous design metrics

Design metric A measurable feature of a system’s implementation Optimizing design metrics is a key challenge

Design challenge – optimizing design metrics

Unit cost: the monetary cost of manufacturing each copy of the system, excluding NRE cost

NRE cost (Non-Recurring Engineering cost): The one-time monetary cost of designing the system

Size: the physical space required by the system Performance: the execution time or throughput of the system Power: the amount of power consumed by the system Flexibility: the ability to change the functionality of the system without

incurring heavy NRE cost Time-to-prototype: the time needed to build a working version of the system Time-to-market: the time required to develop a system to the point that it can

be released and sold to customers Maintainability: the ability to modify the system after its initial release Correctness, safety, many more

Design metric competition -- improving one may worsen others

Expertise with both software and hardware is needed to optimize design metrics Not just a hardware or software

expert, as it is common A designer must be comfortable with

various technologies in order to choose the best for a given application and constraints

SizePerformance

Power

NRE cost

Microcontroller

CCD preprocessor Pixel coprocessorA2D

D2A

JPEG codec

DMA controller

Memory controller ISA bus interface UART LCD ctrl

Display ctrl

Multiplier/Accum

Digital camera chip

lens

CCD

Hardware

Software

Time-to-market: a demanding design metric

Time required to develop a product to the point it can be sold to customers Most demanding metric!

Market window Period during which the product

would have highest sales

Average time-to-market constraint is about 8 months

Delays can be costly IC technology improves allowing

additional capacity, along with customer demand for more functionality Designers need to do even more in even less time.

Revenues ($)

Time (months)

Losses due to delayed market entry

Simplified revenue model Product life = 2W, peak at W Time of market entry defines a

triangle, representing market penetration

Triangle area equals revenue

Loss The difference between the on-

time and delayed triangle areas

On-time Delayedentry entry

Peak revenue

Peak revenue from delayed entry

Market rise Market fall

W 2W

Time

D

On-time

Delayed

Rev

enue

s ($

)

Assume market rise and fall at 45°

Losses due to delayed market entry (cont.)

Area = 1/2 * base * height On-time = 1/2 * 2W * W Delayed = 1/2 * (W-D+W)*(W-D)

Percentage revenue loss = (D(3W-D)/2W2)*100%

Try some examples

– Lifetime 2W=52 wks, delay D=4 wks– (4*(3*26 –4)/2*26^2) = 22%– Lifetime 2W=52 wks, delay D=10 wks– (10*(3*26 –10)/2*26^2) = 50%– Delays are costly!

On-time Delayedentry entry

Peak revenue

Peak revenue from delayed entry

Market rise Market fall

W 2W

Time

D

On-time

Delayed

Rev

enue

s ($

)

Assume market rise and fall at 45°

NRE and unit cost metrics

Costs: Unit cost: the monetary cost of manufacturing each copy of the system, excluding

NRE cost NRE cost (Non-Recurring Engineering cost): The one-time monetary cost of

designing the system total cost = NRE cost + unit cost * # of units per-product cost = total cost / # of units

= (NRE cost / # of units) + unit cost

• Example– NRE=$2000, unit=$100– For 10 units

– total cost = $2000 + 10*$100 = $3000– per-product cost = $2000/10 + $100 = $300

Amortizing NRE cost over the units results in an additional $200 per unit

NRE and unit cost metrics

Compare technologies by costs -- best depends on quantity Technology A: NRE=$2,000, unit=$100 Technology B: NRE=$30,000, unit=$30 Technology C: NRE=$100,000, unit=$2

• But, must also consider time-to-market

The performance design metric

Widely-used measure of system, widely-abused Clock frequency, instructions per second – not good measures Digital camera example – a user cares about how fast it processes images, not clock

speed or instructions per second

Latency (response time) Time between task start and end e.g., Camera A process images in 0.25 seconds

Throughput Tasks per second, e.g. Camera A processes 4 images per second Throughput can be more than latency seems to imply due to concurrency, e.g. Camera

B may process 8 images per second (by capturing a new image while previous image is being stored).

Speedup of B over S = B’s performance / A’s performance Throughput speedup = 8/4 = 2

Performance: A measure of how long the system takes to complete its task

Three key embedded system technologies

Technology A manner of accomplishing a task, especially using technical processes,

methods, or knowledge

Three key technologies for embedded systems1. Processor technology

General purpose processor - Software Single purpose processor – Hardware Application specific software – Peripherals

2. IC technology

3. Design technology

1.Processor technology

The architecture of the computation engine used to implement a system’s desired functionality

Processor does not have to be programmable “Processor” not equal to general-purpose processor

Application-specific

Registers

CustomALU

DatapathController

Program memory

Assembly code for:

total = 0 for i =1 to …

Control logic and State register

Datamemory

IR PC

Single-purpose (“hardware”)

DatapathController

Control logic

State register

Datamemory

index

total

+

IR PC

Registerfile

GeneralALU

DatapathController

Program memory

Assembly code for:

total = 0 for i =1 to …

Control logic and

State register

Datamemory

General-purpose (“software”)

Processor technology

Processors vary in their customization for the problem at hand

total = 0for i = 1 to N loop total += M[i]end loop

General-purpose processor

Single-purpose processor

Application-specific processor

Desired functionality

General-purpose processors

Programmable device used in a variety of applications Also known as “microprocessor”

Features Designer simply writes S/W for off-the-shelf Ps Program in memory General datapath with large register file and

general ALU

User benefits Low time-to-market and NRE costs High flexibility, low unit cost at low quantities

Drawbacks High unit cost for large quantities Unnecessarily large size/power unused Ps res.

“Pentium” the most well-known, but there are hundreds of others

IR PC

Registerfile

GeneralALU

DatapathController

Program memory

Assembly code for:

total = 0 for i =1 to …

Control logic and

State register

Datamemory

IR: Instruction registerPC: Program counter

Single-purpose processors

Digital circuit designed to execute exactly one program – hardware implementation of program a.k.a. coprocessor, accelerator or peripheral Generally, all components other than Ps/ Cs are

single purpose processors.

Features Contains only the components needed to execute a

single program No program memory

Benefits Fast Low power Small size

DatapathController

Control logic

State register

Datamemory

index

total

+

DrawbacksHigh NRE cost, long design timeLow flexibilityLimited performance

(Program hardwired)

Application-specific processors

Programmable processor optimized for a particular class of applications having common characteristics Compromise between general-purpose and single-

purpose processors

Features Program memory Optimized datapath Special functional units

Benefits Some flexibility, good performance, size and power

Examples: Microcontrollers Digital signal processors (DSP)

IR PC

Registers

CustomALU

DatapathController

Program memory

Assembly code for:

total = 0 for i =1 to …

Control logic and

State register

Datamemory

2. IC technology

The manner in which a digital (gate-level) implementation is mapped onto an IC IC: Integrated circuit, or “chip” IC technologies differ in their customization to a design (TTL, CMOS, etc.) IC’s consist of numerous layers (perhaps 10 or more)

• Bottom layer transistors; middle layers logic components; top layer connect them with wires

source drainchannel

oxide

gate

Silicon substrate

IC package IC

IC technology

Three types of IC technologies Full-custom/VLSI Semi-custom ASIC (gate array and standard cell) PLD (Programmable Logic Device)

Full-custom/VLSI

All layers are optimized for an embedded system’s particular digital implementation Placing transistors Sizing transistors Routing wires

Benefits Excellent performance, small size, low power

Drawbacks High NRE cost (e.g., $300k), long time-to-market

Semi-custom / ASIC

Lower layers are fully or partially built Designers are left with routing of wires and maybe placing some blocks

Benefits Good performance, good size, less NRE cost than a full-custom

implementation (perhaps $10k to $100k)

Drawbacks Still require weeks to months to develop

PLD (Programmable Logic Device)

All layers already exist Designers can purchase a previously built IC Connections on the IC are either created or destroyed to implement desired

functionality PLA, PAL, Field-Programmable Gate Array (FPGA) very popular

Benefits Low NRE costs, almost instant IC availability

Drawbacks Bigger, expensive (perhaps $30 per unit), power hungry, slower

Moore’s law

The most important trend in embedded systems Predicted in 1965 by Intel co-founder Gordon Moore

IC transistor capacity has doubled roughly every 18 months for the past several decades

10,000

1,000

100

10

1

0.1

0.01

0.001

Log

ic tr

ansi

stor

s pe

r ch

ip(i

n m

illi

ons)

1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009

Note: logarithmic scale

This trend is predicted to continue for another decade !!!

Graphical illustration of Moore’s law

1981 1984 1987 1990 1993 1996 1999 2002

Leading edgechip in 1981

10,000transistors

Leading edgechip in 2002

150,000,000transistors

Something that doubles frequently grows more quickly than most people realize! A 2002 chip can hold about 15,000 1981 chips inside itself

Design productivity exponential increase

Productivity: Number of transistors one designer can produce / month Exponential increase over the past few decades This productivity increase is in part due to synthesis tools, re-useable libraries, etc.

100,000

10,000

1,000

100

10

1

0.1

0.01

19831981 1987 1989 1991 19931985 1995 1997 1999 2001 2003 2005 2007 2009

Productivity(K) Trans./Staff – Mo.

100 transistors/month 5000 transistors/month

3. The Design TechnologyThe co-design ladder

In the past: Hardware and software design

technologies were very different Recent maturation of synthesis

enables a unified view of hardware and software

Hardware/software “codesign”

Implementation

Assembly instructions

Machine instructions

Register transfers

Compilers(1960's,1970's)

Assemblers, linkers(1950's, 1960's)

Behavioral synthesis(1990's)

RT synthesis(1980's, 1990's)

Logic synthesis(1970's, 1980's)

Microprocessor plus program bits: “software”

VLSI, ASIC, or PLD implementation: “hardware”

Logic gates

Logic equations / FSM's

Sequential program code (e.g., C, VHDL)

The choice of hardware versus software for a particular function is simply a tradeoff among various design metrics, like performance, power, size, NRE cost, and especially flexibility; there is no

fundamental difference between what hardware or software can implement.

Independence of processor and IC technologies

Basic tradeoff General vs. custom With respect to processor technology or IC technology The two technologies are independent Any processor technology can be implemented in any IC technology!

General-purpose

processor

ASIPSingle-purpose

processor

Semi-customPLD Full-custom

General,providing improved:

Customized, providing improved:

Power efficiencyPerformance

SizeCost (high volume)

FlexibilityMaintainability

NRE costTime- to-prototype

Time-to-marketCost (low volume)

Multiple processors of different types on a single chip System on a chip

Design productivity gap

While designer productivity has grown at an impressive rate over the past decades, the rate of improvement has not kept pace with chip capacity

10,000

1,000

100

10

1

0.1

0.01

0.001

Log

ic tr

ansi

stor

s pe

r ch

ip(i

n m

illi

ons)

100,000

10,000

1000

100

10

1

0.1

0.01

Pro

duct

ivit

y(K

) T

rans

./Sta

ff-M

o.

1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009

IC capacity

productivity

Gap

Design productivity gap

1981 leading edge chip required 100 designer months 10,000 transistors / 100 transistors/month

2002 leading edge chip requires 30,000 designer months 150,000,000 / 5000 transistors/month

Designer cost increase from $1M to $300M

10,000

1,000

100

10

1

0.1

0.01

0.001

Log

ic tr

ansi

stor

s pe

r ch

ip(i

n m

illi

ons)

100,000

10,000

1000

100

10

1

0.1

0.01

Pro

duct

ivit

y(K

) T

rans

./Sta

ff-M

o.

1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009

IC capacity

productivity

Gap

The mythical man-month

The situation is even worse than the productivity gap indicates In theory, adding designers to team reduces project completion time In reality, productivity per designer decreases due to complexities of team management

and communication In the software community, known as “the mythical man-month” (Brooks 1975) At some point, can actually lengthen project completion time! (“Too many cooks”)

10 20 30 400

10000

20000

30000

40000

50000

60000

43

24

1916 15 16

18

23

Team

Individual

Months until completion

Number of designers

1M transistors, 1 designer=5000 trans/month

Each additional designer reduces for 100 trans/month

So 2 designers produce 4900 trans/month each

Solution…?

Indeed, the gap between designer productivity and IC capacity goes from bad to worse… We cannot keep adding designers to design more complicated chips, even if we had all the money in the world!

Solution…? New design technologies Train designers in more than one area to be more efficient Need designers who are expert in both hardware and software

This is what this class is about !

Summary

Embedded systems are everywhere Key challenge: optimization of design metrics

Design metrics compete with one another

A unified view of hardware and software is necessary to improve productivity Three key technologies

Processor: general-purpose, application-specific, single-purpose IC: Full-custom, semi-custom, PLD Design technologies